1. Field of the Invention
This invention relates to a method and apparatus for quantifying oxygen contained in a sample gas. The method and apparatus are especially suited for measuring a small amount of oxygen with high accuracy.
II. Background of the Invention
The small amount of oxygen in an industrial gas such as nitrogen, hydrogen, argon and helium is conventionally measured by reacting the oxygen in a sample gas with vapor of yellow phosphorus in a reaction chamber to cause a light-emitting reaction between the oxygen and the vapor of yellow phosphorus. The intensity of the light resulting from the reaction is measured by a photodetector such as photomultiplier tube. The vapor of the yellow phosphorus is obtained by sublimating solid phosphorus at room temperature (15.degree. C. to 25.degree. C.) in the reaction chamber. Since the light intensity of the light emitted by the reaction in the reaction chamber is proportional to the amount of oxygen in the sample gas, the amount of oxygen may be determined by measuring the light intensity of the light emitted by the reaction.
By the conventional method, oxygen may be quantified accurately if the amount of the oxygen is on the order of several ppm to 100 ppm. It is now demanded, however, in the field of, for example, manufacturing of semiconductor devices, that the oxygen in the amount of 1 ppm or less be quantified accurately.
However, the oxygen in the amount of 1 ppm or less may not be quantified accurately since the light intensity is very weak and the photoelectric current supplied from the photodetector is very small accordingly, so that the difference between the intensity of the photoelectric current supplied from the photodetector and the dark current of the photodetector (electric current supplied from the photodetector in the absolute absence of light) becomes small.
Thus, a method by which oxygen in the amount of 1 ppm or less may be quantified accurately is needed.